1. Field of the Invention
The present invention relates to a developing member such as a developing roller used in an electrophotographic image forming apparatus such as a copying machine or a laser printer, and to an electrophotographic image forming apparatus using the developing member.
2. Description of the Related Art
Conventionally, a pressure-developing method has been known as one of the developing methods employed in electrophotographic apparatuses such as copying machines, printers, and facsimile receivers. The pressure-developing method uses a developer of a nonmagnetic toner component and attaches the developer to a latent image on a photosensitive drum to visualize the latent image.
The pressure-developing method has been used in a wide variety of applications because of its advantages such as: magnetic material is not required; the apparatus can be made simple or compact with ease; and a developer can be made up as a color toner with ease.
In an electrophotographic image forming apparatus employing the pressure-developing method, a rotating photosensitive drum is uniformly charged by a charging member and is then irradiated with laser light to form an electrostatic latent image. Subsequently, a developing device supplies a developer on the electrostatic latent image to obtain a toner image, followed by transferring the toner image from the photosensitive drum to a transfer material (recording material). Finally, the toner image on the transfer material is fixed thereon by heating or the like.
After transferring the toner image, the surface of the photosensitive drum is de-charged to clean off any remaining developer. Thus, it comes to be in a waiting state in which it is ready for new image formation.
The above developing device includes: a storage container for storing a developer; a developing member typified by a developing roller, which is arranged so as to be partly exposed with the opening of the container being closed; and a developer-supplying roller, which applies the developer on the surface of a developing roller.
The developing device may be further provided with a developing blade for uniformly forming a thin layer of the developer on the surface of the developing roller by the developer-supplying roller. The thin-film developer can be transferred to an exposure part of the developing roller as the developing roller rotates.
The thin-film developer is attached to an electrostatic latent image on a photosensitive drum, which is disposed opposite to the exposure part of the developing roller to visualize the electrostatic latent image, thereby forming a toner image on the photosensitive drum.
In many cases, for the surface of a developing roller used in such a developing device, a polyurethane resin is used for improving electrostatic properties of toner. However, the polyurethane resin shows a high water-absorbing property. Thus, in such a developing member, the amount of the electrostatic charge of the toner is decreased at high temperature and high humidity in some cases. To solve the above problem, Japanese Patent Application Laid-Open No. H11-212354 (U.S. Pat. No. 6,360,069) discloses a technology to inhibit the amount of the electrostatic charge of the toner from decreasing at high temperature and high humidity, specifically at a temperature of 35° C. and a humidity of 85% RH (relative humidity). That is, a constitution is proposed in which the principle component of the surface layer is a polyurethane resin obtained by reaction of a polyol compound with an isocyanate compound or with the isocyanate compound and a chain extender wherein either or both of the polyol and the chain extender include a polysiloxane skeleton.
Further, in Japanese Patent Application Laid-Open No. 2003-167398 (U.S. Patent Application Publication No. US-2003-0118372) proposes the use of a conductive member as an outermost resin layer, where the conductive member is made of a resin material that contains a urethane resin and a polysiloxane component. It is aimed at preventing the occurrence of fogging at the initial stage and after extensive operation while stably attaining the adhesion of a photosensitive to meet the needs of high image quality, speed-up, and high durability.
An idea of combining a urethane resin and polysiloxane as described above may be effective in realization of a charging member with high performance. However, as the need for a highly reliable developing roller is growing, it may become very difficult for image characteristics under opposite extreme environmental conditions such as a high temperature and high humidity (30° C./85% RH) environment and a low temperature and low humidity (15° C./10% RH) environment to be consistent with each other. Specifically, in a high temperature and high humidity environment, fogging may occur due to remarkable deterioration in frictional chargeability for the developer. In order to solve such a problem, if frictional chargeability is enhanced, the effect of suppressing fogging is obtained. In this case, however, when the developing roller is used in a low temperature and low humidity environment, ghosts may noticeably occur due to the charge-up of the developer ascribable to high frictional chargeability. Since this relation is a reverse correlation, a developing member having more stable chargeability in both a high temperature and high humidity environment and a low temperature and low humidity environment has been hardly realized.